The present invention relates generally to the field of power-driven landing gear equipment for semi-trailers, outriggers, crane stabilizers and the like, and more particularly to a means for effectively limiting the extension and retraction of the powered landing gear legs so as to prevent mechanical failure of the acme jack screw and other leg components due to power overload and over-torque.
At full extension, the acme screw travel in most current landing gear systems is stopped by a welded nut, pin or other mechanical stop to prevent the screw from being driven completely out of the nut. If one continues to fully extend the landing gear the stop nut or pin can fail. This in particularly true of 2-speed heavy lifting landing gear (˜50,000 lb. lift capacity as commonly used on most on-highway trailers) which has a high gear ratio and delivers as much as 14,000 inch pounds of torque to the acme drive screw.
Overextending the landing gear does not just put the acme screw mechanism at risk of failure, but it can also cause severe failure of the leg due to buckling from over-extension. The upper and lower telescoping leg assemblies of the landing gear are designed with a specified overlap at maximum extension. Overextending the leg effectively reduces this “safety” overlap, which can cause catastrophic failure due to leg buckling. In this type of failure, buckling of the landing gear can actually result in the entire trailer collapsing to the ground.
Similarly, at full retraction the acme screw drive nut located at the top of the lower leg assembly will bottom out against the stop plate of the upper leg assembly. If the acme drive screw is driven beyond its fully retracted position, the drive nut can strip, the acme drive screw can strip, or both can fail together resulting in an inoperable landing gear. In the worst case, this can result in the leg collapsing as well (with stripped threads the upper leg assembly is no longer supported by the threads and can collapse along the length of the acme screw). However, a more common failure is for the nut and screw to weld together under friction resulting in a leg that is inoperable (cannot be retracted or extended). This can also be the case at full extension, where the stop locking weld, washer or pin causes the threads to begin to strip and weld the screw and nut together, thereby resulting in the landing gear being inoperable.
The above-described failure modes are typically not as common in manual landing gear systems because of human operator fatigue. That is, the physical effort to continue cranking in the low speed mode beyond what is needed to lift the trailer for hooking or unhooking prevents operators from trying to fully extend the landing gear. In addition, most landing gear have markings painted on the lower leg indicating “Stop”, “Alto” or “Halt” so as to warn the operator not to overextend the legs. It is believed, however, that with the increased use of hydraulic powered drive systems, these failure modes will become more common and more severe. Since safety is a primary objective of advancing the art (prevent injuries from manual cranking) preventing any additional potential failures and safety concerns is of primary concern. Given that an external power source is used and human effort to manually crank the landing gear is eliminated, the risk to over-extend or drive beyond the fully retracted and extended travel stop limits is increased and must be managed.
There have been ongoing efforts to develop a solution for the problem of limiting the end of travel of powered landing gear legs so as to prevent damage to the mechanical components thereof. A number of methods have heretofore been considered and evaluated for resolving the above problem. For instance, general pressure relief valves, with and without foam/rubber cushion stops, have been considered to protect the jacking screw and other components from mechanical failure due to over-torque or over-loading conditions. However, with pressure relief valves there is always an inherent problem that relief valves are unable to react fast enough to stop any spike in pressure and do not provide a reliable means to limit torque. The added elastomer cushion does dampen this spike, but ultimately this solution fails because it cannot be successfully implemented to prevent overload at the end of each travel stop.
Other efforts to resolve the above problem have included the implementation of a pressure switch to bypass or disable the powered drive unit from the landing gear. However, this system adds further complications as electric power needs to be provided and the pressure switch needs to be connected to the power unit or a cross-port valve. Moreover, of added concern is the fact that the pressure switch reaction time is still not fast enough to avoid over-loading the landing gear at both the fully extended and fully retracted travel stop limits. Still further, there is not a reliable way to stop the landing gear at the desired extended position, as it is desired to maintain mechanical over-lap of the upper and lower leg assemblies to provide strength and prevent the legs from failure (e.g., buckling) due to over-extension.
Still further efforts to resolve the above problem have included employing limit switches to trigger a bypass function or disable the powered drive unit of the landing gear. While this system does have merit, it requires the introduction of electrical connections and additional logic for reverse direction operation, as any solution to the above problem requires disabling of the powered drive unit in only one direction, without complete disablement of the drive unit. Such a solution could be costly to implement. It is also possible that an encoder could be utilized to count the number of revolutions of the acme screw and stop turning prior to the end of the stop limit, but here again, implementation of such a system could prove to be costly and add undue complexity to the system as well as require a PLC computer to provide proper control logic to disable the unit in one direction only (one must be able to extend from the fully retracted position or drive the unit to retract from the fully extended position).
Therefore, it is evident there is a substantial and unsatisfied need in the industry for a reliable and cost-effective solution to the problem of limiting the end of travel of powered landing gear legs so as to prevent damage to the mechanical components thereof and enhance safety for the operators of such equipment. Such a system desirably should be relatively simple and cost effective in construction yet capable of automated operation to avoid operator error. Moreover, such a system must overcome the problems of travel limit overload and the limitations inherent with conventional relief valve or pressure switch systems, as well as other issues associated with the aforementioned systems above, and remain capable of disabling travel in one direction while permitting travel in the reverse direction.